The invention is related to the field of energy harvesting, and in particular to improving energy harvesting efficiency of piezoelectric harvesters.
Energy harvesting is an area of growing importance to reduce the dependence of handheld, portable and implantable electronics on batteries. Harvesting ambient vibration energy through piezoelectric means is a popular energy harvesting technique which can supply 100's of μW of available power. A piezoelectric element converts mechanical energy in the form of vibrations into electrical energy and vice-versa. It can be modeled as a current source in parallel with a complex impedance.
Ambient mechanical vibrations produce an AC current in the piezoelectric element which must be rectified to get a DC voltage output. Conventional rectifiers use a full-bridge rectifier 2 or a voltage doubler 10 circuit as shown in FIGS. 1 and 2. The problem with a simple full-bridge strategy is that most of the available power of the harvester 4 is just wasted in charging and discharging the input capacitor.
A piezoelectric harvester is usually represented electrically as a current source in parallel with a capacitor (CP) and resistor (RP). The current source provides current proportional to the input vibration amplitude. For the sake of the following analysis, the input vibrations are assumed to be sinusoidal in nature and hence the current is represented as:IHAR=Ip sin ωt  (1)Some of the prior art in extracting electrical power from piezoelectric harvesters 2, 10 is shown in FIGS. 1 and 2. FIG. 1 shows a full-bridge rectifier and FIG. 2 shows a rectifier 10 which also acts as a voltage doubler. The analysis also assumes ideal diodes 6, 14. The electrical power that is extractable from the full-bridge rectifier using the circuit shown in FIG. 1 is given by:
                              P          RECT                =                                            2              ⁢                                                          ⁢                              V                RECT                                      π                    ⁢                      (                                          I                p                            -                                                V                  RECT                                ⁢                ω                ⁢                                                                  ⁢                                  C                  p                                                      )                                              (        2        )            where VRECT is the rectified output voltage of the full-bridge rectifier 2. The extracted power varies with the output voltage and reaches a maximum at
                              V                      RECT            ,            max                          =                              I            p                                2            ⁢                                                  ⁢            ω            ⁢                                                  ⁢                          C              p                                                          (        3        )            where the maximum power extractable is
                              P                      RECT            ,            max                          =                              I            p            2                                2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            ω            ⁢                                                  ⁢                          C              p                                                          (        4        )            For the voltage doubler case, while the maximum power extractable remains the same, the output voltage (VRECT) at which this is achieved is twice the value as given by eq. (3).
The main limitation of the full-bridge rectifier 2 is that, most of the current available from the harvester 4 does not go into the output at high voltages. This is because, the current first has to go into the capacitor Cp to charge it up to VRECT before the current can go into the output. This happens every time current changes direction from positive to negative and vice-versa. In each of those occasions, the voltage across Cp has to change from +VRECT to −VRECT or from −VRECT to +VRECT. This loss in charge due to charging and discharging of Cp limits the maximum power that can be extracted using the full-bridge rectifier.
Following the rectifier, additional DC-DC converters are required to regulate the output of the rectifier to its maximum power point and to efficiently transfer the energy obtained to the load circuits. These converters can be inductor-based to achieve high efficiency.